error if reducing over empty collection

[simeonschaub]

MWE:

julia> using FoldsCUDA, Folds

julia> Folds.foreach(identity, 1:0, CUDAEx())
ERROR: DomainError with 0:
`x` must be positive.
Stacktrace:
  [1] nextpow(a::Int64, x::Int64)
    @ Base ./intfuncs.jl:457
  [2] _transduce!(buf::Nothing, rf::Transducers.Reduction{Transducers.MapSplat{typeof(identity)}, Transducers.BottomRF{typeof(Folds.Implementations.return_nothing)}}, init::Nothing, arrays::UnitRange{Int64})
    @ FoldsCUDA ~/.julia/packages/FoldsCUDA/Mo35m/src/kernels.jl:123
  [3] transduce_impl(rf::Transducers.Reduction{Transducers.MapSplat{typeof(identity)}, Transducers.BottomRF{typeof(Folds.Implementations.return_nothing)}}, init::Nothing, arrays::UnitRange{Int64})
    @ FoldsCUDA ~/.julia/packages/FoldsCUDA/Mo35m/src/kernels.jl:32
  [4] _transduce_cuda(op::Function, init::Nothing, xs::Base.Iterators.Zip{Tuple{UnitRange{Int64}}})
    @ FoldsCUDA ~/.julia/packages/FoldsCUDA/Mo35m/src/kernels.jl:18
  [5] #_transduce_cuda#5
    @ ~/.julia/packages/FoldsCUDA/Mo35m/src/kernels.jl:1 [inlined]
  [6] _transduce_cuda
    @ ~/.julia/packages/FoldsCUDA/Mo35m/src/kernels.jl:1 [inlined]
  [7] transduce(xf::Transducers.MapSplat{typeof(identity)}, rf::typeof(Folds.Implementations.return_nothing), init::Nothing, xs::Base.Iterators.Zip{Tuple{UnitRange{Int64}}}, exc::CUDAEx{NamedTuple{(), Tuple{}}})
    @ FoldsCUDA ~/.julia/packages/FoldsCUDA/Mo35m/src/api.jl:45
  [8] foreach(f::Function, itr::UnitRange{Int64}, itrs::CUDAEx{NamedTuple{(), Tuple{}}}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Folds.Implementations ~/.julia/packages/Folds/ZayPF/src/reduce.jl:89
  [9] foreach(f::Function, itr::UnitRange{Int64}, itrs::CUDAEx{NamedTuple{(), Tuple{}}})
    @ Folds.Implementations ~/.julia/packages/Folds/ZayPF/src/reduce.jl:77
 [10] top-level scope
    @ REPL[9]:1
 [11] top-level scope
    @ ~/.julia/packages/CUDA/Ey3w2/src/initialization.jl:52

Seems like

FoldsCUDA.jl/src/kernels.jl

Line 124 in bc96774

wanted_threads = nextpow(2, n)

just needs a special case for n == 0